This invention relates to improvements in motor control circuits, and in particular to pulse width modulation (PWM) control of multiple phase brushless motors in electric power assisted steering systems.
Control systems for PWM controlled electric motors, especially DC motors, generally need to measure the current flowing through the windings or phases of the motor and this can either be done by means of separate current sensors for each of the phases, or by means of a single current sensor that is placed in the circuit so as to measure the total instantaneous current flowing between a D.C. power supply and the bridge circuit and motor combination. In a single current sensor system, the multiple motor phase currents are derived by offsetting the PWM patterns of the switches which apply the required voltage to each phase, and sampling the current sensor at appropriate points.
The value of the current demanded in an electric power assisted steering system is generated as a function of the torque demanded from the motor. The torque demand signal is a principally a measure of the amount of torque the motor should produce at a given time.
The measured currents are typically converted into a rotating d-q frame which rotates with the rotor, and then combined with the current demand signal, also in the d-q frame, indicative of the current that is demanded from the motor, to produce a current error signal.
The error signal represents the difference between the current that is demanded in order to achieve a desired torque and the actual current flowing in the motor. The error signal is fed to a current controller which produces a set a voltage demand signal, also typically in the d-q frame, representative of the voltage to be applied to each phase of the motor that will best drive the error signal towards zero. The d-q voltage signal is then converted into PWM signals for the motor phases depending on which PWM strategy is used. The controller therefore acts to vary the PWM phase voltages in order to try to constantly minimise the magnitude of the error signal thereby ensuring that the motor current is as close as possible to the demanded current.
In a practical system the current controller will comprise a PI or PID or other type of feedback controller. The function of the current controller is to modify the voltages applied to the motor with an aim of keeping the error signal value as small as possible. The controller forms a closed loop.
Motors are used in a wide range of application, and one particular application relevant to this invention is electric power assisted steering systems. In a typical electric power assisted steering system an electric motor is connected to a steering column or steering rack. The torque applied to the steering column by a driver turning a steering wheel is measured or estimated.
Around the current controller the demand signal is produced by a torque controller, which forms another closed loop with a measurement or estimate of the torque in the steering downstream, of the motor as one input and the torque applied to the steering wheel as another. The controller calculates a demand signal that is indicative of an amount of assistance torque that is required from the motor to help the driver turn the wheel. For example, if the motor applies a torque that turns the column in the same direction as the driver applied torque it will have the effect of making the steering easier to turn. The value of the demand signal is calculated by the torque controller using an appropriate algorithm and many different algorithms are known in the art. For example, one algorithm that is incorporated herein by reference is disclosed in patent application (reference). The demand signal may be expressed in terms of a torque value, or in terms of currents in the d-q frame of reference.
Motor drive circuits using feedback control and PWM are well known in the art. For example WO2006005927, discloses a typical system and the teaching of that document is incorporated herein by reference. The general layout of the control system is shown in FIG. 2 of the drawings.